Generally, the rotary frame of a conventional spinning type fishing reel is rotated by a drive mechanism comprising a handle, a tubular main shaft having a master gear, and a headed screw shaft for coupling the main shaft with the handle, the tubular main shaft being journalled to cylindrical bosses at the side walls of the reel body, the screw shaft being inserted through the main shaft and screwed at its screw thread with a threaded bore at a handle shaft to couple the handle with the main shaft, so that when the handle is turned to rotate the main shaft the rotary frame is driven through the master gear and a pinion provided at the rotary frame and engageable with the master gear.
Such screwable coupling of the main shaft with the handle, even when firmly tightened, becomes loose due to vibrations caused when the handle turns or the line is cast, and leads to an unsteady turning of the handle, whereby the screw shaft should be periodically retightened.